This invention relates to a receiver for FM/FMX broadcasting. More specifically, this invention relates to a FMX ID (identification) signal detection apparatus for detecting an ID signal identifying a received broadcasting as not a FM but FMX broadcasting.
As is already known, in a FM stereophonic broadcasting, stereophonically related audio frequency source signals L (left) and R (right) are converted into a sum signal M (L+R) and a difference signal S (L-R). Then, the signal M is converted into a frequency in the region of 50 Hz to 15 KHz, while the signal S 23 to 53 KHz by sub-carrier system so as to be broadcasted, respectively.
In the case of frequency modulation, as is already known, the noise spectral characteristics includes so-called triangular noise in which the higher the modulated frequency, the more the noise increases. Therefore, in a weak electric field state such as long distance reception, larger noise is generated in the signal S than M, the frequency of the former being higher than the latter. This results in more noise in the stereo broadcasting than monoral bloadcasting.
Accordingly, in the FMX broadcastinn, noise is reduced by employing a FMX difference signal SD which is obtained b compressing the dynamic range of the signal S.
The signal SD is broadcasted by modulating a sub-carrier 90 degrees out of phase with that for the signal S. Furthermore, an ID signal for identifying the FMX broadcasting is added to the signal SD by modulating the signal SD with a signal of 9.986 Hz with modulation level of 1%. These signals M, S, SD and a pilot signal constitute a FMX composite signal.
Such as U.S. Pat. No. 4,485,483 issued Nov. 27, 1984 to E. Torick, et al. discloses the basic principle of a FMX broadcasting system detail.
FIG. 1 shows conventional apparatus for detecting the ID signal. The FMX composite signal is applied to a terminal 20 by a receiving means not shown. The terminal 20 is connected to a demodulator 22 at its input. The demodulator 22 at its output where the signal SD is produced is directly connected to a mode/dematrix circuit 28 at its imput and also connected thereto at its other input through a band pass filter 24 and an ID detection circuit 26.
On the other hand, the demodulator 22 at its other output where the signals M and S are produced is connected to the mode/dematrix circuit 28 at its input to which the signal SD is directly connected. The mode/dematrix circuit 28 produces a demodulated sound signal at its output and its detection output is connected to a FMX indicator 30 which is applied with a drive voltage Vcc.
When the FMX composite signal is received, it is demodulated by the demodulator 22 and the signals M, S and SD are produced therefrom, respectively. The signal SD is applied to the band pass filter 24 which picks a frequency component corresponding to the ID signal from the signal SD. The frequency component is applied to the ID detection circuit 26 where the level of the frequency component is examined to detect the ID signal.
When the ID signal is detected, it is acknowledged by the mode/dematrix circuit 28. The mode/dematrix circuit 28 switches the demodulated sound signal to a FMX signal based on the acknowledgement and the FMX indicator 30 is turned on. Accordingly, it is indicated that the FMX broadcasting is now received.
However, it is difficult to narrow a frequency band width of the band pass filter 24. Thus, there is a problem that the detection circuit often malfunctions when a low frequency noise is superimposed on the signal SD. Namely, even though the FM broadcasting is now received, the mode/dematrix circuit 28 erroneously switches the demodulated sound signal to the FMX signal.